Thursday, March 27, 2014

New paper finds natural variability accounts for > 50% of long-term temperature change in many regions over past century

A paper published today in Climate Dynamics finds natural multidecadal variability accounts for "more than 30% of long-term temperature variation" in "most regions" and "more than 50% in parts of North America, East Asia, Northern Eurasia, Northern Africa and Greenland" over the past century. The authors find natural ocean oscillations the Atlantic Multidecadal Oscillation [AMO] and Pacific Decadal Oscillation [PDO] account for "more than 40% of the amplitude" of the natural multidecadal variability.

The authors find the well known natural 60-year climate cycle associated with the PDO is present in mid-high-latitude lands, whereas a 20 to 30 year cycle [one-third to one-half the 60 year cycle] is present in low-latitude lands.

The characteristics of multidecadal variability (MDV) in global land surface air temperature (SAT) are analyzed based on observations. The role of sea surface temperature (SST) variations in generating MDV in land SAT is assessed using atmospheric general circulation model simulations forced by observed SST. MDV in land SAT exhibits regional differences, with amplitude larger than 0.3 °C mainly over North America, East Asia, Northern Eurasia, Northern Africa and Greenland for the study period of 1902–2004. MDV can account for more than 30 % of long-term temperature variation during the last century in most regions, especially more than 50 % in parts of the above-mentioned regions. The SST-forced simulations reproduce the observed feature of zonal mean MDV in land SAT, though with weaker amplitude especially at the northern high-latitudes. Two types of MDV in land SAT, one of 60-year-timescale, mainly observed in the northern mid-high-latitude lands, and another of 20–30-year-timescale, mainly observed in the low-latitude lands, are also well reproduced. The SST-forced MDV accounts for more than 40 % amplitude of observed MDV in most regions. Except for some sporadically distributed regions in central Eurasia, South America and Western Australia, the SST-forced multidecadal variations are well in-phase with observations. The Atlantic Multidecadal Oscillation and Pacific Decadal Oscillation signals are found dominant in MDV of both the observed and SST-forced land SAT, suggesting important roles of these oceanic oscillations in generating MDV in global land SAT.

10 comments:

What is the cause of the ~60 year quasi-cycle? I noticed this just a little while ago.

The Earth's rotation axis is nutatiing with a period of about 18.6 years. However, that is only part of the story. In actual fact, the nutation takes the form of an elliptical cone, as shown here. The distance between the J2000 polar axis and the actual rotation axis looks like this. Its period is necessarily halved, to about 9.3 years.

Thus, the magnitude of the component of the magnetic moment of the Sun along the Earth's rotation axis should have periods of about

T1 = 11*9.3/(11+9.3) = 5 years

T2 = 11*9.3/(11-9.3) = 60 years

Coincidence? Maybe. But, is there not a 5-ish year quasi-periodicity to the major temperature sets? Hard to say for sure, but there surely are several ups and downs which are in the neighborhood of 5 years.

I didn't really give the "hole" much more thought. Mainly, it just told me I didn't need to worry that the gap invalidated the idea of GHGs as radiators which cool the globe under convection. The gap does not generally indicate atmospheric attenuation because it already exists at the ocean surface. -Bart

Doesn't the ocean look blue to you when you look at it from the shore? If it looked yellow at the surface, and looked blue from space, then there would be a definite indication that the atmosphere was taking the red out.

What we would really need to evaluate atmospheric transmission would be the difference between a spectrum at the surface, and at TOA. -Bart

I have been upbraided in another location that the distribution is almost totally in the infrared, and the criticism is apt. The blue of the oceans is reflected sunlight, not thermal emission. It doesn't look blue at night.

So, mea culpa. I was hasty.

It is still true, however, that TOA alone does not tell us the transmission from surface to there. I do not know what the spectrum near the surface is. Maybe I will read over your links and see if they shed any light on this. -Bart

I think that the answer is simply this: a gap in surface to TOA transmission obviously means that some upward bound radiation is being retained. However, that does not mean that the gap is monotonic, i.e., that it is incrementally increasing with an incremental increase in greenhouse gases in all climate states. This leaves the baseline GHE in effect to warm the surface beyond what it otherwise would be, but says that there is a limit to how much it can do so.

I think a lot of problems with climate science result from an overreliance on linearized models. This may be such a case. - Bart